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Evolution
Lecture 20, Chapter 13
Topics for today:
Evolutionary response to selection in quantitative traits
1. How many genes?
2. How do polygenic traits evolve?
3. How strong is natural selection?
What is a quantitative trait?
What traits are not polygenic?
Examples of polygenic traits in humans
How many loci contribute to polygenic are traits?
QTL Mapping (Quantitative trait loci)
Mimulus lewisii
Mimulus cardinalis
Basic requirements for this technique
 Two different phenotypes (in this case separate species)
 Genetic map available for both species
 Many molecular markers for each species with known locations on the genetic
map
Basic steps to identify QTLs
1. Cross to the F1 – all intermediate
2. Cross to the F2 - recombination causes variation to segregation
3. Find associations between the phenotypic attributes and the molecular markers
4. If an individual expresses the phenotype and also possesses the marker, then
you know that a gene that influences the trait resides in that region of
chromosome.
Fig 13.1new 13.5old
What regions of chromosome 3 affect bristle number in Drosophila?
 Artificial selection for high and low bristle number
 Many additive QTL
 Some strong epistatic effects
Number of QTLs found
 33 sternopleural (thorax)
 31 abdominal
 11 pleiotropic
Fig. 13.2 new 13.6old
(QTL review paper linked on web)
QTLs are well-described for agricultural species
 Example of QTLs that are associated with salt tolerance in rice
 Can find an interaction QTL viewer for cows at
http://bovineqtlv2.tamu.edu/index.html
How do polygenic traits evolve?
 R = h2NS
 R = Response to selection = change in a trait between generations
 h2N = Narrow-sense heritability
 S = Strength of selection = change within a generation
What is heritability?
 Broad-sense heritability:
h2B = VG = VA + VD + VI
VP VA + VD + VI + VE

Narrow sense heritability:

Only the variance attributable to additive genes is transmitted across
generations
Gene interactions, including dominance (subscript D) and epistasis (subscript I),
are created anew depending upon the specific combination of alleles that
offspring acquire from their parents

h2N = VA = VA
VP VA + VD + VI + VE
Heritability can be estimated by artificial selection
 R = h2NS
 S is determined experimentally
 R is observed
 h2realized = R
S
Fig. 13.4 new 13.8old
How do you measure selection?
Two measurements
1. i = intensity of selection (before and after selection)
= za –zb
VP
2. b = partial regression coefficient between relative fitness and trait (effect holding
all other traits constant)
Strength of selection measured on beak size
 No reproduction in drought year
 Strength of selection = change within generations
Fig 13.7 new 13.11 old
Strength of selection
 Estimates from many studies
 Generally weak
 Fecundity & mating > survival
 Even weak selection can have strong effects over time
Fig 13.14 old only